This invention relates to a method and apparatus for tuning a broadband filter and in particular to an automatic tuning system for a broadband filter.
In the prior art there are many approximate methods employed for tuning of broadband filters. In general, a broadband filter has an input resonator, an output resonator and a coupling capacitor. The input resonator includes a primary winding of a transformer that is resonated by a variable capacitor, Ci; and the output resonator includes the secondary winding of the transformer which is resonated by a second variable capacitor, Co. An impedance matching element is provided by a variable coupling capacitor, Cc. The general prior art method of tuning the resonator usually provided for shorting Co, or greatly mistuning the output resonator, and adjusting Ci until a series resonance (phasing zero) is obtained at the filter's input. Phasing zero is defined as the condition in which the imaginary component of the measured impedance of the filter's input is zero. Alternatively, Ci may be adjusted for a parallel resonance (phasing zero) at the filter's input also. Other methods provide for adjusting Ci until the absolute value of the input resonator's impedance is at a selected resistance such as 50 ohms. There are still other methods which try to minimize through adjustment of Ci the reflected power at the input terminal of the broadband filter in conjunction with a high loss attenuator. All of the above prior art methods are inexact, complicated and difficult to implement especially in the situation where the tuning of the broadband filter is performed automatically and without manual intervention.